High voltage (HV) battery chargers, with an input voltage Vin in the range of 20V, typically make use of inductor based power converters which achieve conversion efficiency in the range of about 90%. Such a relatively low efficiency is due to the fact that the efficiency of an inductive step-down power converter is optimum for output voltages Vout which are only slightly lower than the input voltage Vin, i.e. for conversion ratios Vout/Vin˜1.
The battery or output voltage is typically Vout=3.6V (i.e. <4.2V) and the input voltage Vin (also referred to herein as the transfer voltage) from an external power supply may be up to 20V. As a result of this, the Vin/Vout conversion ratio is relatively high and the efficiency is reduced. One parameter affecting the efficiency of an inductive step-down converter is the switching frequency, wherein the efficiency of the power converter is typically increased, when the switching frequency is reduced. On the other hand, a reasonable current ripple at relatively low switching frequencies typically requires inductors with relatively high inductance. The size of an inductor typically increases with increasing inductance. Hence, the use of inductors with relatively high inductance is typically incompatible with modern portable electronic devices such as thin tablet PCs or smartphones. Consequently, battery chargers for portable electronic devices usually make use of relatively low inductance coils, thereby triggering relatively high switching frequencies and by that limiting the achievable maximum conversion efficiency of the power converter of the battery charger.